The present invention relates to an exhaust gas purification apparatus.
In the past, in a diesel engine, particulate contained in the exhaust gas has been removed by arranging a particulate filter in the engine exhaust passage, using that particulate filter to trap the particulate in the exhaust gas once, and igniting and burning the particulate trapped on the particulate filter to regenerate the particulate filter. In this case, making the trapped particulate ignite and burn requires a considerably high temperature and a considerable time.
On the other hand, there is known an internal combustion engine carrying on the particulate filter an NOx absorbent which absorbs NOx when the air-fuel ratio is lean and releases the absorbed NOx when the air-fuel ratio is made rich (see Japanese Unexamined Patent Publication (Kokai) No. 6-159037). In this internal combustion engine, the engine is usually operated under a lean air-fuel ratio. When the amount of NOx absorbed in the NOx absorbent exceeds an allowable value, the air-fuel ratio is temporarily made rich to cause the NOx to be released from the NOx absorbent.
When NOx is released from the NOx absorbent and reduced, the heat occurring at the time of reduction of the NOx causes the temperature of the particulate filter to rise. Therefore, in one example of this internal combustion engine, when the NOx finishes being released, the air-fuel ratio is returned to the lean state once again. Using the fact that the temperature of the particulate filter is rising at that time, the particulate deposited on the particulate filter is made to burn. Further, in another example, when NOx should be released from the NOx absorbent, when the exhaust pressure upstream of the particulate filter does not exceed a predetermined pressure, the air-fuel ratio is just made rich, while when the exhaust pressure upstream of the particulate filter exceeds the predetermined pressure, the air-fuel ratio is made rich to cause the NOx to be released from the NOx absorbent, then the air-fuel ratio is made lean to cause the particulate deposited on the particulate filter to burn.
As explained above, however, causing the particulate trapped on the particulate filter to ignite and burn requires a considerably high temperature and a considerable time. In this case, getting the temperature of the particulate filter to rise to the temperature where the deposited particulate will ignite and burn requires that energy be supplied from the outside. Therefore, usually, additional fuel is fed or an electric heater is used to cause the temperature of the particulate filter to rise. Therefore, if burning of the particulate takes time, that much more excess energy will become required. Reduction of this excess consumption of energy requires that the time needed for burning the particulate be shortened as much as possible.
Inventors researched the properties of the deposited particulate from this viewpoint and as a result the properties of the deposited particulate gradually became clear. The details will be explained later, but briefly, it was found that the longer the time of deposition of the particulate on the particulate filter, the harder the deposited particulate will be to oxidize and as a result ignition and burning of the deposited particulate required a considerably high temperature and considerable time. That is, it was found that if it were possible to change the particulate in property to a property of easy oxidation when the deposited particulate becomes hard to oxidize, the time required for burning the particulate can be shortened.
Therefore, the inventors proceeded with further research on this point and as a result found that by temporarily making the air-fuel ratio rich, the deposited particulate could be changed in property to a property of easy oxidation. That is, it was found that if making the air-fuel ratio temporarily rich when the deposited particulate becomes hard to oxidize, the particulate becomes easy to oxidize and therefore the time required for burning the particulate can be shortened.
In the above-mentioned known internal combustion engine, NOx is released from the NOx absorbent by occasionally temporarily making the air-fuel ratio rich. Therefore, the pattern of change of the air-fuel ratio resembles that of the present invention. In this known internal combustion engine, however, the air-fuel ratio is temporarily made rich when the amount of NOx absorption of the NOx absorbent exceeds an allowable amount, while in the present invention, the air-fuel ratio is made temporarily rich when the deposited particulate becomes hard to oxidize. Not only are the objects of making the air-fuel ratios rich different, but also the timings of making them rich differ. That is, even if making the air-fuel ratio rich at the timing of releasing NOx from the NOx absorbent, it is not necessarily possible to continue to change the deposited particulate to an easily oxidizing state.
An object of the present invention is to provide an exhaust gas purification apparatus enabling particulate deposited on a particulate filter to be made to burn in a short time.
According to a first aspect of the present invention, there is provided an exhaust gas purification apparatus of an internal combustion engine in which a particulate filter for trapping and removing particulate in an exhaust gas is arranged in an engine exhaust passage and in which burning is continued under a lean air-fuel ratio, the apparatus comprising predicting means for predicting if the particulate deposited on the particulate filter has changed in property to a property harder to oxidize compared with right after deposition, air-fuel ratio switching means for temporarily switching the air-fuel ratio of exhaust gas flowing into the particulate filter from lean to rich to cause the particulate deposited on the particulate filter to change in property to a property of easy oxidation when it is predicted that the particulate deposited on the particulate filter has changed in property to a property harder to oxidize compared with right after deposition, judging means for judging if the amount of particulate deposited on the particulate filter has exceeded a predetermined amount, and temperature control means for causing the temperature of the particulate filter to rise under a lean air-fuel ratio so as to remove by oxidation the particulate deposited on the particulate filter when the amount of the particulate deposited on the particulate filter has exceeded a predetermined amount.
According to a second aspect of the present invention, there is provided an exhaust gas purification apparatus of an internal combustion engine in which a particulate filter for trapping and removing particulate in an exhaust gas is arranged in an engine exhaust passage and in which burning is continued under a lean air-fuel ratio, the apparatus comprising first judging means for judging if the particulate deposited on the particulate filter has changed in property to a property harder to oxidize compared with right after deposition, air-fuel ratio switching means for temporarily switching the air-fuel ratio of exhaust gas flowing into the particulate filter from lean to rich to cause the particulate deposited on the particulate filter to change in property to a property of easy oxidation when it is judged that the particulate deposited on the particulate filter has changed in property to a property harder to oxidize compared with right after deposition, second judging means for judging if the amount of particulate deposited on the particulate filter has exceeded a predetermined amount, and temperature control means for causing the temperature of the particulate filter to rise under a lean air-fuel ratio so as to remove by oxidation the particulate deposited on the particulate filter when the amount of the particulate deposited on the particulate filter has exceeded a predetermined amount.
According to a third aspect of the present invention, there is provided an exhaust gas purification apparatus of an internal combustion engine in which a particulate filter for trapping and removing particulate in an exhaust gas is arranged in an engine exhaust passage and in which burning is continued under a lean air-fuel ratio, the apparatus comprising air-fuel ratio switching means able to temporarily switch the air-fuel ratio of exhaust gas flowing into the particulate filter from lean to rich, judging means for judging if the amount of particulate deposited on the particulate filter has exceeded a predetermined amount, and temperature control means for causing the temperature of the particulate filter to rise under a lean air-fuel ratio so as to remove by oxidation the particulate deposited on the particulate filter after temporarily switching the air-fuel ratio of the exhaust gas flowing into the particulate filter from lean to rich to make the particulate deposited on the particulate filter change in property to a property of easy oxidation when the amount of the particulate deposited on the particulate filter has exceeded a predetermined amount.